Efficient materials and low energy manufacturing techniques are needed for the production of conductive patterns. Direct printing of conductive materials has garnered interest from researchers and industry in the past few years. The conductive ink market is expected to exceed $3 billion in the next few years in a wide array of end point uses including antennas, RFID tags, photovoltaics, flexible electronics and displays. The advantages stem from the cost savings over traditional techniques such as vacuum deposition and photolithography. The reduction in costs are reflected by large area scalability, efficient energy and materials usage, and the availability of existing manufacturing capacity.
Solution phase inks and pastes are typically composed of metallic particles suspended in an organic solvent or binder. Silver in the form of flakes is the most predominant material used in the direct printing of conductive lines. The photovoltaic industry is expected to become a major consumer of silver for conductive current collectors that are typically deposited using screen printing techniques and sintered using thermal processing. There have been a number of applications utilizing silver nanoparticles in inkjet formulations and sintering using lower temperature methods such as inert gas plasmas, microwaves, and intense pulsed light (IPL). These help reduce overall costs, but such devices still rely on a relatively expensive silver.
Inexpensive materials such as copper that utilize the lower temperature sintering processes above could further reduce the costs associated with conductive patterns. Copper and silver have very similar electrical conductivity and copper is significantly less expensive. However, silver is commonly used in printed electronics because it is stable in air, whereas copper tends to oxidize during the sintering process, which significantly reduces conductivity. Copper inks have been developed that utilize reducing capping agents on pure copper nanoparticles to produce conductive patterns at temperatures between 200° C. and 320° C. using inert gas plasmas. However, these dispersions rely on pure copper nanoparticles using relatively complex processes that inevitably add costs.
In addition, because copper nanoparticles oxidize under ambient conditions, the presence of oxides on the surface can result in the need for higher processing temperatures as well as significant reductions in conductivity. Techniques for synthesizing copper nanoparticles include electrochemical deposition, hydrothermal methods, electrolysis, microwave assisted polyol methods, reverse micellar synthesis, sonochemical methods, thermal reduction, and thermal decomposition of copper oxalate, which leads to the formation of the powder form of copper rather than an ink to make conductive films. In some instances the instability of copper even under atmospheric conditions commonly results in the use of organic stabilizers such as poly(N-vinyl-pyrrolidone) to reduce the copper oxide on the surface of the nanoparticles during the intense pulsed light process.
Hence, there remains a need for methods to overcome the shortcomings of known deposition techniques as well as known copper compositions that are prone to oxidation. There also remains a need for cost-effective materials and methods for applying conductive film patterns onto a substrate which can also resist oxidation.